This invention relates to a capacitance level sensor for sensing the level of a viscous, electrically conducting material in a tank.
In a prior capacitance sensing device shown in Townsend application Ser. No. 93,392, now U.S. Pat. No. 4,347,740, entitled "Capacitive Level Sensing Device" and assigned to the assignee of this application, an astable multivibrator develops an output signal having a period dependent upon the capacitance of a variable capacitor and the resistance of a charging resistor. Where the capacitor is a level measuring probe, the capacitance is dependent upon the level of material in a tank. In the Townsend circuit the resistance of the charging resistor is large compared to the resistance of the material within the track. For example, the charging resistor may have a value in the range of 10 kilohms. In the case of low viscosity, electrically insulative materials, this circuit provides an accurate indication of the level of the material within the tank. However, when one attempts to sense the level of a material which is electrically conductive and viscous so that a coating can build up on the probe, false readings can occur.